Known image scanners may be equipped with a plurality of one-dimensional contact type image sensors such as CCDs. FIG. 5 shows a case in which the reading is executed by an image scanner equipped with two image sensors A and B. In the main scanning direction of line 1, the sensor A reads 0, 1, 2 . . . , and (N-1) sequentially, and the sensor B reads N, N+1, N+2, . . . , and M-1 sequentially. When the reading of the line 1 is completed, the two sensors A and B together read lines 2, 3, 4, . . . and so on in the auxiliary scanning direction while sharing their functions as in line 1. The video signals thus read out are converted into binary signals, for example, and are stored in a buffer memory (not shown) until they are outputted to a signal processor (not shown) where they are subjected to predetermined signal processing.
Image scanner of the prior art, however, can read the lines with a clock frequency as high as 100 nsec if they use a read element having a high sensitivity such as a CCD as its image sensor, but must read with a clock frequency as low as 250 nsec when using a read element having a lower sensitivity such as an amorphous silicon element. Such a relatively low scanning rate in the case of a low sensitivity read element is, of course, undesirable.